Cross coupling in a two-axis control system for stabilized platforms

Sammanfattning: Inertial stabilized platforms consisting of a two-axis gimbal assembly are often modelled as two independent SISO systems, describing the dynamics of the elevation axis and the azimuth axis respectively. In reality the state of the elevation channel and the state of the azimuth channel affect each other. Hence, the system is better modelled as a MIMO system with coupled dynamics, which means that the system has multiple inputs and outputs, where each input can affect multiple outputs. Since the couplings between the elevation channel and the azimuth channel have a deteriorating effect on control it is of interest to analyse what gives rise to the coupled dynamics and if control performance can be improved by considering the coupled dynamics. For this purpose, this thesis attempts to derive a dynamic model of the system of interest, both with the aid of physical modeling and system identification. Both modeling methods result in models with similar dynamics which seem to capture the coupled dynamics in the relevant frequency range. From the physical modeling it can be inferred that the degree of coupled dynamics depends on the mass distribution of the two-axis gimbal assembly. For the specific configuration of the system used in this investigation, the degree of coupled dynamics proved to be relatively small with relatively small impact on control. Based on the derived models, three types of controllers were implemented, decentralized control, decentralized control with a decoupler and decentralized control with an inner loop for rejection of mutual disturbances acting between the elevation axis and azimuth axis. Compared to standard decentralized control, the decoupler resulted in a somewhat better reference tracking and in a somewhat worsened disturbance rejection. Compared to standard decentralized control, the inner loop disturbance compensator resulted in a somewhat better performance for reference and disturbance rejection.